(i) Field of the Invention
The present invention relates to carbon fibers for reinforcement of concrete, and more specifically, it relates to mesophase pitch-based carbon fibers for reinforcement of concrete which have excellent adhesive properties to cement, good process passage properties through a direct spray gun and good bundling properties and which are particularly suitable for a direct spray method.
(ii) Description of the Prior Art
In recent years, carbon fibers have been used not only as materials in various fields of, for example, airplane parts, automobile parts, sporting goods and the like, but also as reinforcement materials for resins and cements, because of having excellent features such as high strength, high modulus of elasticity and light weight. Thus, the demand of the carbon fibers has been remarkably increased.
The carbon fibers can be roughly classified into polyacrylonitrile-based (PAN-based) carbon fibers and pitch-based carbon fibers. The former PAN-based carbon fibers are high-performance (HP) carbon fibers which can be obtained from a polyacrylonitrile as a raw material, and they usually have high strength and moderate modulus of elasticity. However, the PAN-based carbon fibers have some drawbacks. That is, acrylonitrile fibers which are raw materials are expensive, and the yield of the carbon fibers from these fibers is extremely low, 45% or less. Furthermore, since these acrylonitrile fibers are produced by a wet spinning method, intricate steps are required to manufacture strands consisting of a small number of filaments, which unavoidably increases manufacturing costs.
On the other hand, the pitch-based carbon fibers can be obtained from a raw material such as a petroleum pitch or a coal tar pitch, and the pitch-based carbon fibers have some merits. That is, the pitch which is the raw material is inexpensive and abundantly available, and the yield of the carbon fibers is high.
In order to improve physical properties, various researches have been conducted, and nowadays, it has been successfully attained to obtain high-performance (HP) carbon fibers having high strength and high modulus of elasticity from a mesophase (liquid crystals) pitch containing an optically anisotropic phase, a neomesophase pitch, a pre-mesophase pitch or a latent anisotropic pitch. In this connection, when an optically isotropic pitch is used, general-purpose (GP) carbon fibers having low strength and low modulus of elasticity can be only obtained.
In the meantime, kneaded materials of hydraulic cement powders have been widely used as various building materials and civil engineering materials. In order to reinforce such a kneaded material and to prevent the occurrence of cracks, it has been heretofore attempted to blend a fibrous material with the kneaded material. However, the employment of asbestos as the reinforcement fibers is not preferable, because the asbestos is a carcinogen. In addition, glass fibers are poor in alkali resistance, so that the strength of the glass fibers themselves inconveniently deteriorates in the cement. Thus, various kinds of organic fibers and alkali-resistant glass fibers are instead used as the reinforcement materials. However, the organic fibers are poor in fire resistance, and even in the case of the alkali-resistant glass fibers, the strength of the kneaded material declines, when these fibers are used for a long period of time under alkaline circumstances of the cement or the like. Hence, much attention is now paid to carbon fibers having excellent heat resistance and excellent chemical resistance as well as high strength and high modulus of elasticity.
However, when added to and mixed with the cement, the carbon fibers are poorer in adhesive properties (or stickiness) to the cement as compared with the asbestos and the glass fibers, and therefore there is the problem that they cannot exert a sufficient effect as the reinforcement material. Accordingly, various contrivances have been made in order to increase the adhesive properties of the carbon fibers to the cement and to thereby heighten the strength of the resultant carbon fibers-reinforced cement material. For example, there have been suggested a method in which carbon fiber strands impregnated with a hydrophobic liquid resin such as an epoxy resin are stretched in cement, and the resin and the cement are then simultaneously hardened (Japanese Patent Publication No. 19620/1983), a method in which carbon fibers mutually bound by a water-soluble binder such as methyl cellulose are arranged in one direction or two crossable directions in cement (Japanese Patent Application Laid-open No. 129657/1981), a method in which cement slurry layers are superposed upon each other with the interposition of a carbon fiber sheet including a water-soluble synthetic resin emulsion such as an acrylic emulsion (Japanese Patent Application Laid-open No. 223659/1983), a method in which in manufacturing a reinforced cement material by a hacek manufacturing method, a nonionic or a cationic high polymeric coagulant such as a polyalkylamino acrylate is applied onto the surfaces of fibers (Japanese Patent Application Laid-open No. 81052/1985), and a fibrous material for cement reinforcement in which a cationic rubber latex is applied onto the surfaces of carbon fibers (Japanese Patent Application Laid-open No. 108755/1987).
In these conventional techniques, however, restriction is put on the morphology of the carbon fibers to be used and a construction method to be selected, and the kind of cement to be used is limited. In addition, they have the problem that the adhesive properties to cement are not sufficient. For these reasons, the conventional techniques are not always sufficiently satisfactory.
In recent years, in the manufacture of a concrete composite material containing a certain kind of fibers as a reinforcement material, a direct spray method in which longer fibers (25 to 30 mm or more) can be used and which can sufficiently exert dynamic characteristics has been noticed and put to practical use. In the case that this direct spray method is applied, it is important that the carbon fibers for reinforcement are excellent in adhesive properties to cement, and that they have good process passage properties through a direct spray gun, i.e., friction between the carbon fibers and metals is so low as to make the carbon fibers easily slidable.